1. Field of the Invention
The present invention relates generally to semiconductor memory devices and methods for making such devices. More particularly, this invention relates to a phase-change memory (PCM) device utilizing a diode-selected array and a self-aligned method for fabricating the same.
2. Description of the Prior Art
Phase-change memory (PCM) is a type of non-volatile computer memory, which uses the unique behavior of phase-change materials such as chalcogenide glass. With the application of heat, the chalcogenide glass can be switched between two states, i.e., crystalline and amorphous states. The crystalline and amorphous states of chalcogenide glass have dramatically different electrical resistivity, and this forms the basis by which data are stored.
FIG. 1 is a schematic diagram showing a cross-sectional view of a conventional phase-change memory cell structure. As shown in FIG. 1, the phase-change memory cell structure includes a silicon substrate 10 with a bottom electrode 12 thereon. A dielectric layer 14 is formed over the bottom electrode 12 and a heating electrode 16 is formed in the dielectric layer 14. A patterned phase-change material layer 20 is provided on the dielectric layer 14. The patterned phase-change material layer 20 may be formed in a dielectric layer 18. A bottom surface of the phase-change material layer 20 partially contacts the heating electrode 16. A dielectric layer 24 is formed over the dielectric layer 18 and a top electrode 22 is formed over and in the dielectric layer 24, thereby contacting the phase-change material layer 20. During memory cell operation, a large amount of current flows through the heating electrode 16 to heat up an interface between the phase-change material layer 20 and the heating electrode 16, thereby transforming the phase of the phase-change material layer 20.
Currently, to enhance applications of phase-change memory devices, size of the memory cells of the phase change memory devices is being required to be further reduced. With size reduction of the memory cell, however, it also means working current of the memory cells should also be reduced while increasing memory cell density. One challenge for current phase-change memory technology has been the requirement of high programming current density in the active volume for switching the state of the phase-change material during a write operation. One approach is reducing the contact surface area between the heating electrode 16 and the phase change material layer 20, such as through reducing a diameter Do of the heating electrode 16, thereby maintaining adequate current density at the interface. However, diameter scalability of the heating electrode 16 is limited by ability of current photolithography.
The contact between the hot phase-change region and the adjacent dielectric is another fundamental concern. The dielectric may begin to leak current at higher temperature, or may lose adhesion when expanding at a different rate from the phase-change material.